This has been an interesting period for planetary science. In the last post, I mentioned the landing of Philae on a comet. As an update, unfortunately all has not gone well. The comet landed well, it bounced, and ended up in a shady spot, do most of what it has managed has relied on batteries. We do not know yet what data it has sent back, so we have no real idea on how successful the venture was, but from my point of view, the news is less good. In my last post, I mentioned that I would like to see what was encased in the ice. What happened was that Philae left it to the last to drill down (because they were afraid that the action of the drill might launch Philae back off the comet, as its gravity is very weak) and they wanted to do as much as they could before that risk was taken. They drilled, but apparently the drill hit something very hard, and when they withdrew the drill and tried to analyze its core, it appeared that there was no sample inside the drill. This is one of the curses of this sort of work. When designing some form of robot, you have to guess exactly what conditions you will meet.
However, a most interesting image has also been released by the European Space Agency. The star, HL Tauri has been found with an accretion disk around it. The star is about 1 million years old, and the disk has rings in it, with dark gaps between them. The most obvious cause for such rings would be the formation of planets, although that does not mean there is a planet in every gap, because while a planet will clear out dust on its path, gravitational resonance will also clear out material. Gravitational resonance is a term for when the orbital period at a given position is an exact multiple/fraction of another. Thus if the planet had a period, say, 12 years (roughly Jupiter’s “year”) there would be 2:1 resonances at a distance where the orbital period was 24 years, or at a distance where the orbital period was 6 years. Where this happens, over a period of time the various gravitational effects, instead of cancelling and circularizing, tend to reinforce and the bigger object causes the very much smaller one to change orbit.
So, are there planets there? One answer is, we don’t know because we have not seen them. Up to a point, this is a bit of a negative in this case. At first sight it may seem obvious that we would not see planets because they are too dull, but that is not the case with very newly formed giants. Thus there is a star HR 8799 and we can see four giant planets around it. The reason we can see them is that they are newly-formed giants, and when they take up the gas, the gravitational energy of the gas falling onto the planet heats it to a yellow-white heat, and the planets glow relatively brightly. Given we cannot see planets here, but we can see the disk, what does that mean?
One obvious thing that it can mean is that planets have yet to get big enough to glow brightly. In my theory of planetary formation (Planetary Formation and Biogenesis) our star had to have formed its planets by about 1 million years. The reason for this assessment is that there is a star LkCa 15 that is 3 million years old, and it has a planet much bigger than Jupiter, and significantly further from the star. Planetary growth should be faster, the closer to the star, at least for the same sort of planet, because the density of matter increases as it falls into the star. (The circumferences of the orbits decrease, and if the same amount of matter is presence, there much be more per unit volume.) Incidentally, we know about the planet around LkCa 15 because we “see” it, at least in images obtained by powerful telescopes, so it is glowing. Since we only see one giant, my theory requires there to be three other giants we cannot see, presumably because they are yet of insufficient size to glow sufficiently brightly for us to image them. So, if I am right, 1 My gets you giants of the size we have, and the longer the disk lasts, the bigger the giants get.
All of which shows there is still a lot of interest in planetary research